Conventional MSM photodetectors for operation at wavelengths shorter than 900 nm comprise a plurality of metal electrical contacts on a GaAs substrate. An electrical bias is applied between alternate contacts and the substrate is irradiated with an optical signal. The optical signal generates carriers in the substrate which are swept to the contacts by the applied bias where they are detected as a photocurrent.
Carriers which are generated in regions of the substrate which are near to the contacts are subjected to larger electric fields and have a shorter distance to travel to reach the contacts than do carriers which are generated deep in the substrate farther from the contacts. Thus, carriers generated near to the contacts will reach the contacts and be detected sooner than carriers generated deep in the substrate. Consequently a short, sharp pulse in the optical signal will produce a longer, less sharp pulse in the photocurrent. This effect limits the useful operating speed or bandwidth of conventional MSM photodetectors for operation at wavelengths shorter than 900 nm.